1. Field of the Invention
The invention pertains generally to air/fuel ratio controllers for internal combustion engines and is more particularly directed to closed loop systems utilizing integral control.
2. Prior Art
Open loop air/fuel ratio schedulers were developed as a means of providing the precision injection timing and regulation needed to control electromagnetic fuel injectors in electronic fuel injection systems. This precise regulation of electronic fuel injection systems is necessary for the reduction of noxious emissions and for the economization of fuel.
The open loop scheduler receives a plurality of engine operating parameters for various sensors such as manifold absolute pressure MAP, RPM, air temperature, coolant temperature, etc. These engine parameters describe the amount of fuel that is required to be injected for the particular operating condition of the engine according to a schedule. The schedule is generally based upon the amount of fuel that is necessary to provide a stoichiometric air/fuel ratio for the mass air flow inducted into the engine. The open loop schedule is a fixed calculation or function developed by careful measurement and data taken from a representative vehicle. It is clear that one schedule will not be able to provide exact stoichiometric operation for all vehicles because of differing tolerances in assembly and different equipment configurations. Moreover, wear and aging will affect certain systems more than others.
Adaptive or closed loop correction is now used to overcome these difficulties in open loop systems. One type of closed loop system used to advantage has been the closed loop O.sub.2 system. This system comprises basically an oxygen sensor detecting the oxygen content of the exhaust gas of the internal combustion engine and an integral controller. The integral controller will respond to the oxygen sensor detecting the presence of oxygen (a lean condition) by increasing the fuel flow factorially and will respond to the detecting the absence of oxygen (a rich condition by decreasing the fuel flow factorially.
A characteristic limit cycle oscillation is thus developed with a stoichiometric air/fuel ratio being the average or base reference. The peak correction provided by the integrator for the limit cycle is determined mainly by the gain or ramp rate of the integral controller and the transport delay which a charge of fuel and air experiences from its induction into the cylinders to its detection at the O.sub.2 sensor as exhaust gas. Generally the limit cycle oscillation has a period of approximately 4.tau. where .tau. is the transport delay time. The peak-to-peak correction of the integral controller is on the order of twice the ramp rate multiplied by the transport lag. The transport lag is inversely proportional to the speed or RPM of the engine in a substantially linear manner.
Although the closed loop O.sub.2 controller provides an advantageous method of correcting the open loop fuel schedule for variations in vehicles, precision limitations of open loop calibration, aging, and wear conditions, there are still some problems with the system dynamics of such a controller.
The amount of system gain and consequently the amount of correction of such a system is a tradeoff between transient response and quiescent response. At steady state conditions, constant load or RPM, the gain of such a system should be small as a large integrator ramp rate will introduce torque roll and an unevenness in the engine performance. With these steady state conditions present, ramp rate (gain) and authority should be enough to just correct for the aging factors to keep the system in calibration.
This low gain while providing excellent quiescent correction is much to slow for transient responses where a relatively large change in air/fuel ratio may be needed immediately or operating conditions have changed the fuel requirements far from the original operating point.
Thus, many present closed loop O.sub.2 systems use a gain rate that is slower than that desired for transients but faster than that desired for steady state. This is not a solution to the problem but merely a compromise between what is desirable and what is considered an operational system.
There is one system disclosed in U.S. Pat. No. 3,782,347 issued to Schmidt et al that attempts to solve this problem by switching the integration rate of the controller from one fixed rate to a faster fixed rate in response to the O.sub.2 sensor remaining in one state for a set period of time. This system will overshoot small transients just outside the timing range because of the high gain rate it switches to once the time period has elapsed. It may take a number of cycles to return to steady state in a worst case condition because of the uni-directional gain rate correction.
Another system disclosed in U.S. Pat. No. 3,831,564 changes an integral controller gain rate in response to an operating parameter of the engine. The method, however, does not allow the closed loop O.sub.2 system to return to a steady state condition once a suspected transient has been corrected for and may cause gain rates and authority levels incompatible with smooth system operation. Further, this system will not deliver a high gain rate at a low level of the controlling varible which may be necessary. Such a system would not be advantageous during decelerations where the manifold absolute pressure would be dropping significantly.